A hydraulic excavator is known as a typical construction machine including an upper swing structure. A hydraulic system for such a hydraulic excavator very often uses a pump control unit that incorporates a regulator for controlling a displacement volume of a hydraulic pump to which a torque control function is added. The pump control unit incorporating a regulator to which the torque control function is added guides a delivery pressure of the hydraulic pump to the regulator. When the delivery pressure builds up so that an absorption torque of the hydraulic pump reaches a set maximum absorption torque, the pump control unit controls to reduce the displacement volume of the hydraulic pump for any further increase in the delivery pressure of the hydraulic pump, thereby controls to keep the absorption torque of the hydraulic pump within the set maximum absorption torque. This prevents engine stall due to overload of a prime mover.
If there are two or more hydraulic pumps involved, a pump control unit that performs torque control called total horsepower control is generally employed. The total horsepower control works as follows. For example, as disclosed in patent document 1, a delivery pressure of each of two hydraulic pumps (hereinafter referred to as first and second hydraulic pumps) is guided to a regulator of each of the two hydraulic pumps. When a sum of an absorption torque of the first hydraulic pump and an absorption torque of the second hydraulic pump reaches a set maximum absorption torque, the total horsepower control works to reduce the displacement volume of each of the first and second hydraulic pumps for any further increase in the delivery pressure of the hydraulic pump. This allows total horsepower assigned to the first and second hydraulic pumps to be used, when an actuator involved in each of the first and second hydraulic pumps is independently driven, so that an effective use of a prime mover output can be achieved.
Patent document 2 discloses a pump control unit incorporating two or more hydraulic pumps. When it is determined, based on electrical signals from a plurality of control levers, that work requires two of a plurality of actuators to be operated simultaneously, distribution ratios of an engine output to be distributed to the hydraulic pumps connected to each of the two actuators are set, according to a combination of the two actuators. A tilting angle of each of the hydraulic pumps is controlled to achieve the distribution ratios.